This invention relates generally to illumination and more particularly to an illumination system for use in a photocopying environment for the purpose of tailoring the illumination profile of an object or document to be copied.
In one form of photocopying apparatus in which this invention might be applied, a flat document platen supports an original document for imaging through a projection lens disposed along the optical path between the object platen and a photoreceptive surface disposed at the image plane of the lens. The photoreceptor is typically disposed on a rotating cylindrical drum. Because of the curvature of the photoreceptor drum, a narrow exposure slit is disposed adjacent to the drum and along its length to limit the image field incident on the photoreceptor.
It is desirable to provide uniform exposure across the width of the photoreceptor, that is along the length of the exposure slit. In order to accomplish this, the object illumination must be nonuniform. One reason for this, as is well known, is that light propagating from an object to its image attenuates outwardly from the image center in proportion to the cos.sup.4 .theta., where .theta. is the angle subtended by any spot in the image plane with the optical axis of the system. Another reason why nonuniform document illumination is desirable is the typical use of an exposure slit having other than uniform width along its length. A more thorough discussion of this latter consideration is to be found in application Ser. No. 737,301, .Iadd.now abandoned .Iaddend.filed herewith by Simpson and Rees, the disclosure of which is hereby incorporated by reference in the present specification.
It is an object of this invention to provide an illumination system by which an object is nonuniformly illuminated, thus to provide uniform exposure of the object at its image plane.